A series of experiments designed to determine whether "doubleheadedness" affects the interaction of myosin with actin in a specific manner. To that purpose the effects of both proteins are compared on polymerization, on potentiation and on displacement from rigor complexes. Comparison between the conformation of rigor and force generating complexes using actin-based probes. Study of the mechanisms responsible for the steepness of the Ca switch on regulated actin.